1. Field of the Invention
The invention relates to a switching power source apparatus for controlling an output voltage to a constant voltage by switching an input power source and, more particularly, to a switching power source apparatus which is used for a load such as to temporarily request a large peak current for a stationary output current.
1. Related Background Art
FIG. 5 is a block diagram showing a construction of a conventional switching power source apparatus of a step-down chopper type. In the diagram, reference numerals 501 and 502 denote input terminals. A DC power source Vin is connected between the input terminals 501 and 502 and a DC voltage is supplied. Reference numerals 503 and 504 denote output terminals. A load 505 is connected between the output terminals 503 and 504. Q5 denotes a switching transistor in which a collector is connected to one input terminal 501, an emitter is connected to one terminal of a choke coil L5 and a cathode of a diode D5, and a base is connected to a control circuit 506.
Another terminal of the choke coil L5 is connected to one terminal of a current detecting resistor R5 and a current detecting circuit 507. Another terminal of the resistor R5 is connected to the plus side of a smoothing capacitor C5, the current detecting circuit 507, a voltage detecting circuit 508, and one output terminal 503. An output of the current detecting circuit 507 connected between the current detecting resistor R5 and an output of the voltage detecting circuit 508 connected to one output terminal 503 are connected to the control circuit 506. The other input terminal 502, the other output terminal 504, an anode side of the diode D5, and a cathode side of the smoothing capacitor C5 are commonly connected to the ground.
The operation of the switching power source apparatus of the step-down chopper type of such a construction will now be described.
When the DC power source Vin is supplied from the input terminals 501 and 502, a PWM (pulse width modulation) control is executed by the control circuit 506 on the basis of outputs of the current detecting circuit 507 and voltage detecting circuit 508 and a pulse which is pulse width modulated is supplied to the base of the switching transistor Q5. When the pulse is supplied to the base of the switching transistor Q5, the switching transistor Q5 is turned on/off. For a period of time during which the switching transistor Q5 is ON, energy is supplied to the choke coil L5, smoothing capacitor C5, and load 505. Since current flows, the energy is accumulated in the choke coil L5. In this instance, the diode D5 is OFF.
When the switching transistor Q5 is turned off, the energy accumulated in the choke coil L5 is supplied to the load 505 through the diode D5. The current detecting circuit 507 detects the current flowing in the current detecting resistor R5 and generates a signal to the control circuit 506. The voltage detecting circuit 508 detects the voltage (output voltage) at one output terminal 503 and generates a signal to the control circuit 506. The control circuit 506 changes a duty ratio of the pulse which is supplied to the base of the switching transistor Q5 so that an output voltage is made constant by a signal from the voltage detecting circuit 508. Further, when an overvoltage occurs in the output or an overcurrent flows in the apparatus by a signal from the current detecting circuit 507, the control circuit 506 stops the operation of the switching transistor Q5, thereby keeping the output voltage constant and protecting the apparatus from the overcurrent and overvoltage.
A set value to protect against the overcurrent has to be set to a slightly large value so as to correspond to a peak output current.
It is now assumed that the load 505 in FIG. 5 is, for example, a motor which is used for a paper feed in a copying apparatus or the like such that a sudden rotation is performed after a stop period or a sudden rotation and a stop are repeatedly carried out or, even if the motor is not a paper feed motor, when the motor is suddenly rotated as in a case where a rotational speed suddenly rises from a constant rotating state or the like, a very large peak current is requested for a stationary current (current when the motor is stopped or when it is rotated at a constant speed). For example, it is now assumed that a current as shown in an output current Iout in FIG. 6A is requested in the load (when the motor load is activated from a stop state, an output current suddenly increases and a peak current flows).
Input/output power in FIG. 5 will now be considered. Since the output power suddenly increases, the input power also obviously suddenly increases. In this instance, a peak current such as input current Iin in FIG. 6B also flows to the input side of the switching power source apparatus.
Although an explanation using the drawings is omitted, even in case of a power source apparatus (forward converter, flyback converter, etc.) in which a commercially available AC power source is inputted and AC is rectified and smoothed and switched through a transformer, thereby supplying electric power to the secondary side, when it is used for the load as mentioned above, a peak current flows to the input side.
In the conventional switching power source apparatus, when a sudden peak current is requested in the load (for example, at the time of an activation of the motor load or the like), since a large current also flows to the input side of the switching power source apparatus, as an input power source, a transformer, and circuit elements of the switching power source apparatus, those having large capacities have to be used so that they can cope with the peak current. There is a problem such that the output voltage drops when a temporary abnormality of the input power source (voltage drop, power supply interruption, or the like) occurs.